In nitride light emitting diode (hereinafter LED) structures, the index of refraction mismatch between GaN and air makes light extraction difficult. In particular, light is often reflected back into the LED structure. One technique used to improve the light extraction efficiency from a LED is to roughen the LED surface to minimize reflectivity at the GaN to air interface.
Current methods of roughening a laser surface utilizes exposing the LED surface to a wet or plasma etch. Such an etch is described in Liu, US Patent application number 20080113463 A1 entitled “Method of Fabricating GaN device with a Laser” which is hereby incorporated by reference. The etching process attacks different crystal planes of the LED surface at different rates resulting in faceted structures ranging between 100 nm and 500 nm wide.
Unfortunately, etchants capable of etching into AlGaN are highly corrosive. Furthermore, to achieve the desired result, several minutes (per batch) of exposure to the etchant is needed. The long period of exposure to a highly corrosive etchant requires sealing of exposed metals portions of the LED to prevent damage to those metal surfaces. Examples of typical etchanting methods include ICP dry etching, immersion in heated potassium hydroxide (KOH) or using a Xe lamp in combination with KOH to produce a photoelectrochemical process (PEC) process. One of the most popular etchants, KOH, not only attacks metals but also attacks photoresist, silicon and even silicon oxide making it a difficult etchant to work with.
Thus an improved method of roughening a LED surface that does not require complicated sealing of all exposed surfaces is needed.